The handling of foodstuff put very high demands on the packaging. They must meet the hygienic requirements, i.e. bacteria and flavouring agents should not be able to migrate through the packages to and from the surrounding environment. In some cases the tray should even be gas tight, i.e. for chilled food for long storage or fresh meat in modified atmosphere. They should have enough strength to resist the handling during storage and transport. Tough environmental demands are also put on the packaging, i.a. as to recycling, composting or burning of used packages.
It has become more and more common with ready-cooked food dishes, and in addition to serving as a package for storage of the foodstuff, there are also requirements that it should be possible to put the packages directly into a microwave oven or a conventional oven for cooking or heating of the food dishes.
Packages in the form of aluminium forms are nowadays used to a great extent. They resist conventional ovens, but the disadvantages are that they become very hot and sometimes even impossible to hold in your hands. Aluminium forms are also very fragile and cannot resist a great load. Moreover, they cannot be put in a microwave oven
Another common type of packaging is a tray of foamed, vacuum-formed or casted polyester. An essential disadvantage with solid or foamed plastic trays is that they cannot be put in a conventional oven, since they will then melt. The same will thing also happens with solid plastic trays that are common in convenient stores nowadays.
In U.S. Pat. No. 6,245,199 a method of mould-casting trays, where the starting material is a suspension comprising cellulose fibres, is described. Moulds are dipped, from above, in a bath of the suspension, after which the compression-moulding is performed under heat.
The choice of material suggested in the U.S. patent for the forming pulp is however not optimal for the manufacturing process and results in a formed tray lacking in function. Moreover, there is no specification of the pulp, only how the machinery works. Also, the described manufacturing process and assembling have some flaws like low and uneconomical production rate, large areas that have to be well sealed against air leakage. Air pressure from the back of the moulds demands extremely good rigidity as the tool tolerances, when in contact, are less than 1 mm. This results in bad reproducibility and a decreased quality of the trays. The tools used may also cause crushing of cellulose at certain locations on the tray.
It is known to form trays from a starting material in the form of a paper web normally comprising multiple layers. The forming is performed by stretch-forming the web using a pressing tool. One example of a method of this kind is described in EP 1 160 379-A2. This document suggests the use of a paper web that has been improved as regards its stretchability and elasticity, properties that are important when the material is to be stretched and deformed in order to form it.
The forming of trays from a material web is however associated with a number of disadvantages. Even if the flexibility and elasticity have been improved, as is indicated in EP 1 160 379-A2, there are still limitations with regard to its flexibility and elasticity, which in turn results in limitations in the formability. It is impossible to produce deep trays or multi compartment trays from a web, since it is impossible to form a tray or bowl from a flat sheet even if you have moistened it up to water a content 50%. Furthermore, undesired folds are formed when depressions are made in the material web in connection with the forming of the trays. The web used may even break. An essential disadvantage with the trays according to EP 1 160 379-A2, is that the formed trays are stretched and that they have built-in tensions that may cause the formed trays to be deformed when exposed to stresses in the form of changes in temperature or when exposed to moisture or dampness.